This invention relates to a method and apparatus for generating a dot clock signal for controlling operation of a print head.
In a known type of image transfer ink jet printer, an image receiving drum rotates relative to an ink jet array print head which spans at least a segment of the length of the drum. The drum is driven to rotate about its central axis, so that the print head scans the peripheral surface of the drum. (In the following description, the X-axis is parallel to the axis of rotation of the drum whereas the Y-axis is tangential to the drum and displacement along the Y-axis is measured about the periphery of the drum. References to units of linear displacement relate to linear displacement about the periphery of the drum.) The angular position of the drum is measured by use of a Y-axis position encoder which generates a pulse-form encoder signal at a spatial frequency of 150 cycles per inch (about 59.1 cycles per cm) of peripheral movement of the drum. As the drum rotates, the print head fires and ink drops are ejected toward the drum and are deposited on the transfer drum at a peripheral position that depends on the timing of the firing of the print head relative to the angle of rotation of the drum from a datum position which may be considered to be the top of the image being formed on the transfer drum. When the image has been deposited on the drum, a transfer roller is moved into contact with the transfer drum. A sheet of print medium, such as paper or transparency film, is fed through the nip between the transfer drum and the transfer roller, and the image is transferred from the transfer drum to the print medium.
For full color printing, the ink jet array print head includes an array of yellow jets spaced apart in the X direction and similar arrays of cyan, magenta and black jets. The arrays are spaced apart in the Y direction, so that as the drum rotates, the peripheral surface of the drum is scanned sequentially by the yellow, cyan, magenta and black arrays.
If two adjacent arrays of jets, e.g. the array of cyan jets and the array of magenta jets, are spaced apart in the Y direction by a distance D.sub.min and the other pairs of adjacent arrays of jets are spaced apart in the Y direction by an integer multiple of D.sub.min, the printer will be able to address an integer number of pixels (separately addressable areas on the surface of the transfer drum) in the Y direction between each array of jets if it is able to resolve a displacement D.sub.min in the Y direction into an integer number of pixels.
Firing of the print head is controlled by a dot clock signal. If the distance D.sub.min is 11/300 inch (about .931 mm), a displacement D.sub.min in the Y direction can be resolved into 11 pixels, achieving a vertical resolution of 300 dpi (about 118 dots per cm), by a dot clock signal having a frequency of 300 cycles per inch (about 118 cycles per cm).
A dot clock signal having a frequency of 300 inch.sup.-1 can be generated from an encoder signal having a frequency of 150 inch.sup.-1 by detecting both rising and falling edges of the encoder signal. A dot clock signal having a frequency of 600 inch.sup.-1, which permits a resolution of 600 dpi to be achieved, can be generated by detecting the rising and falling edges of a signal having a frequency of 300 inch.sup.-1.
An encoder signal having a frequency of 218 cycles per inch (about 85.8 cycles per cm) could be used to generate a dot clock signal of 436 inch.sup.-1 (about 172 cm.sup.-1) by detecting both rising and falling edges of the encoder signal. With a print head of the structure described above and having a distance D.sub.min equal to 11/300 inch, a dot clock signal at 436 inch.sup.-1 corresponds almost exactly to 16 pixels between arrays. The signal at 436 inch.sup.-1 could be used to generate a dot clock signal of 872 inch.sup.-1 (about 343 cm.sup.-1), corresponding almost exactly to 32 pixels between arrays.
There are disadvantages to the technique of detecting both rising and falling edges of the encoder signal (or a derivative of the encoder signal) for generating a dot clock signal at a higher frequency.
Further, it would be desirable to have greater flexibility in selecting the vertical resolution with which an image can be printed using an image transfer ink jet printer without its being necessary to employ a different Y-axis position encoder and without limitation to integer multiples of the spatial frequency of the output signal of the Y-axis position encoder.